I'm having a hard time understanding the benefits of VCD (like Slingshot) over a conventional boiler in terms of volume of condensate produced per unit of energy input. I've found quite a few resources explaining (in too much detail for me to fully grasp) the operation of VCDs ([A], [B]), but I haven't come across (or understood maybe) any explaining why compressing the vapor is either necessary or more than marginally advantageous.
Here's what I think I understand: inlet water comes into the system, flows through a heat exchanger to be preheated by outflowing condensate, bringing the inlet water as close to the boiling temp as practical. It's then boiled by heat exchanged from the compressed vapor (above atmospheric pressure and normal boiling/condensing temp), producing low pressure vapor that feeds into the compressor. Let me know if I've got this wrong.
I can see how this would be a closed system and could be very efficient, but it seems to me (erroneously, I assume) that a non-compression boiler could be (very nearly) as efficient. Is the higher temperature difference between the compressed vapor and inlet water the main advantage?
I've seen several sources focus on how the compression stage raises the condensation point. Beyond increasing the temp difference across the heat exchangers, it seems like this would also push the point (or zone) where the vapor condenses further downstream in the heat exchanger and a larger share of its length would be filled with vapor than without compression. Is there an advantage to this? Seems like the lower mass (vapor vs. liquid) would transfer less total heat, even with the larger temp difference.
I'm sure I'm looking at this wrong, but I can't see where. Any thoughts?